The IRS Instructions for Form 8889 allow contributions for 2025 made in 2026 by April 15, 2026, the unextended filing deadline. A tax-return filing extension therefore does not, by itself, extend this contribution window. As of October 1, 2026, the ordinary 2025 deadline has passed.
The instructions provide a later-date exception for people serving in, or supporting, the U.S. Armed Forces in a designated combat zone or contingency operation. This timing rule does not remove HSA eligibility requirements or contribution limits. Check the rules for the contribution year rather than treating the prior-year deadline as permission to contribute any amount.